1. Field of the Invention
The present invention relates to a hydraulic power steering apparatus suitable for use in automobiles and the like.
2. Discussion of Related Art
FIG. 1 shows a conventional hydraulic power steering apparatus in which a bypass control valve is employed to reduce the energy consumption. The power steering apparatus comprises a pump 1, a power cylinder 14, a reservoir 11, and a control valve 13. This control valve 13 is provided with variable throttles V1, V2, V3 and V4 disposed in four passages which are connected to the pump 1, a pair of chambers of the power cylinder 14 and the reservoir 11 and which form a bridge circuit. The power steering apparatus further comprises a flow control valve 4 and a bypass control valve 9.
The flow control valve 4 is composed of a valve spool 4a and a spring 6 disposed in a spring chamber 6a formed at the back of the valve spool 4a. The flow control valve 4 is disposed in a bypass passage 5 and the flow of fluid flowing from an inlet port 5a to an outlet port 5b of the flow control valve 4 is controlled by the valve spool 4a. A connection port 6b of the spring chamber 6a is connected to a supply passage 2 via a control orifice 7 and to the reservoir 11 via a relief valve 8. The valve spool 4a responds to a pressure difference between the upstream side and downstream side of a metering orifice 3 disposed in a supply passage 2 which connects the pump 1 and the control valve 13 so that the bypass passage 5 is opened and closed by the valve spool 4a in accordance with the pressure difference, thereby maintaining the flow rate of operational fluid supplied to the control valve 13 constant. The connection port 6b of the spring chamber 6a is also connected to the reservoir 11 via the bypass control valve 9 which is composed of a control spool 9a having slits 9b at its rear end and a spring 10 disposed at the back of the control spool 9a. The slits 9b form a variable orifice which controls the flow of operational fluid flowing to the reservoir 11. The bypass control valve 9 is also connected to the supply passage 2 so that the back pressure of the control valve 13, i.e., load pressure is lead to the front side of the bypass control valve 9. When the back pressure of the control valve 13 is low, i.e., the control valve 13 is in a neutral state, the control spool 9 is moved to the left as viewed in FIG. 1 so that the variable orifice is fully opened. With this operation, the pressure in the spring chamber 6a of the flow control valve 4 is lowered so that the valve spool 4a is displaced to open the bypass passage 5 much more. As a result, the energy consumed by the pump 1 can be reduced.
The bypass control valve 9, however, has the following drawbacks. When the control valve 13 is operated, a very high pressure (the pressure of operational fluid at the time when discharged from the pump 1) acts on the front end surface of the control spool 9a while an atmospheric pressure (the pressure of operational fluid at the time when sucked by the pump 1) acts on the rear end surface of the control spool 9a. Due to this large pressure difference, the spring 10 must have a large spring constant in the case where it is required to control the control spool 9a within a short stroke. When the spring constant of the spring 10 is made larger, adversary effects caused by variations in the axial position of the spring become large. This hinders accurate operation. Moreover, in order to secure that each spring has the same spring constant, selection of springs is required, thereby increasing production costs.